1. Technical Field
The present invention relates to an electric power steering control system for an automobile and to a control method thereof, in which an assist force when manipulating a steering wheel is generated by means of a motor.
2. Background Art
As a power steering system for an automobile, an electric power steering apparatus has been developed which apparatus detects a vehicle speed and a steering torque acting on a steering shaft, drives a motor by a drive current determined in response to the detected vehicle speed and torque, adds an assist force onto the steering shaft by means of a rotational force of the motor, and then provides a driver a comfortable steering feeling.
Hitherto, in such a type of electric power steering apparatus, as a motor for adding the assist force onto the steering shaft, a DC electric motor with a brush such as a commutator motor has been mainly used. Since the motor is with a brush, however, it is possible that any trouble or abnormal state occurs in the motor itself.
Therefore, recently use of a brushless motor has been proposed instead of the DC electric motor with a brush. However, such a motor and/or a control apparatus come to be complicated and expensive. Thus, to put into practice the control suitable for the power steering apparatus, several inherent problems to be overcome remain in terms of cost or performance.
In the electric power steering apparatus using the brushless motor, at a current command section, a command current value of is determined in response to a detected vehicle speed and torque, a motor drive current is detected by a current detector, a value of a feedback current fed back to a current control section is computed, and an inverter is controlled so as to clear a deviation between said value of the feedback current and said value of current command. In this manner, the brushless motor is PWM driven.
In such a control method, each phase voltage applied to the electric motor forms generally a sine wave. In this case, however, a line output voltage for the normal PMW drive can be only in the range of an amplitude of a fundamental wave thereof being not more than {square root over ( )}{fraction (3/2)} of that of a power supply voltage. Therefore, the voltage is utilized inefficiently.
In the electric power steering control system, the motor is generally driven by a vehicle-mounted battery to generate the steering assist force. A voltage of the battery is not constant, and the voltage varies depending on conditions of use of the battery.
For example, in the electric power steering control system, even in the case that a normal battery voltage of 14V is lowered to a degree of 9V, a normal operation is still required.
In the electric power steering control system in which environment of the power supply is so harsh, it is desirable that the power supply voltage is utilized at the maximum. Efficiency of utilizing the power supply voltage getting worse means that a maximum output of the motor is reduced resulting in lowering in performance of the electric power steering control system.
In the case of applying a sine-wave voltage to each phase of the brushless motor, normal PWM drive can be performed only in the range that amplitude of the fundamental wave of the line output voltage is not more than {square root over ( )}{fraction (3/2)} of the power supply voltage. As a result, efficiency of utilizing the voltage gets worse. Since, in the electric power steering control system, the motor is driven by means of the battery voltage, lowering in efficiency of utilizing the power supply voltage brings about undesirable lowering in performance of the electric power steering control system.
The present invention was made to solve the above-discussed problems, and has an object of providing an electric power steering control system and a control method thereof capable of performing normally any PWM drive even in the range that an amplitude of a fundamental wave of a line output voltage is not more than that of a power supply voltage, thereby utilizing the power supply voltage at the maximum.
An electric power steering control system according to the invention arranged to control a drive current of a brushless motor that generates a steering assist force on the basis of a steering torque acting on a steering shaft and a vehicle speed, comprises:
a d-q-axis current command section for computing a d- and q-axis current command value for generating a steering assist force in response to a predetermined characteristic on the basis of the mentioned steering torque and the mentioned vehicle speed;
an electrical angle computing section for computing an electrical angle signal on the basis of a rotational position signal of the mentioned brushless motor;
a d-q coordinate transformation section for performing a d-q coordinate transformation on the basis of a detection current signal in response to a three-phase AC current to be inputted to the mentioned brushless motor and of the mentioned electrical angle signal, thereby obtaining a d- and q-axis detection current value;
a d-q axis voltage command section for computing a d- and q-axis voltage command value in response to a deviation between the mentioned d- and q-axis current command value and the mentioned d- and q-axis detection current value;
a d-q coordinate inverse transformation section for performing a d-q coordinate inverse transformation on the basis of the mentioned d- and q-axis voltage command value and the mentioned electrical angle signal thereby computing a three-phase voltage command value;
a three-phase voltage command compensation section for performing compensation with respect to the mentioned three-phase voltage command value according to a computing expression for improving efficiency of utilizing a previously set voltage, thereby computing a three-phase voltage command compensation value; and
a PWM output section for generating a PWM waveform for PWM drive of the mentioned brushless motor in response to the mentioned three-phase voltage command compensation value.
As a result of such arrangement, it is possible to obtain an electric power steering control system capable of performing normally any PWM drive even in the range that an amplitude of a fundamental wave of a line output voltage is not more than that of a power supply voltage, thereby utilizing the power supply voltage at the maximum.
It is preferable that, in the mentioned three-phase voltage command compensation section, a three-phase voltage command compensation value is obtained by subtracting a predetermined voltage value from the mentioned three-phase voltage command value.
It is preferable that a value obtained by averaging a maximum value and a minimum value among the mentioned the three-phase voltage command values is used as the mentioned predetermined voltage value.
It is preferable that a minimum value of the mentioned three-phase voltage command values is used as the mentioned predetermined voltage value.
It is preferable that, in the case of using the minimum value of the mentioned three-phase voltage command values as the mentioned predetermined voltage value, compensation is performed with respect to the mentioned three-phase voltage command value in the mentioned three-phase voltage command compensation section, only when revolution of the mentioned brushless motor is not less than a predetermined value.
It is preferable that, in the mentioned three-phase voltage command compensation section, a tertiary higher-harmonic of the mentioned three-phase voltage command value is superposed on the mentioned three-phase voltage command value, thereby obtaining a three-phase voltage command compensation value.
It is preferable that, in the case of obtaining a three-phase voltage command compensation value by superposing the tertiary higher harmonic of the mentioned three-phase voltage command value on the mentioned three-phase voltage command value, compensation is performed with respect to the mentioned three-phase voltage command value in the mentioned three-phase voltage command compensation section, only when the mentioned steering torque is not less than a predetermined value.
It is preferable that, in the mentioned d-q coordinate transformation section, a following mathematical expression is used as a computing expression to perform the d-q coordinate transformation:       [                            Id                                      Iq                      ]    =                    2            ⁡              [                                                            -                                  sin                  ⁡                                      (                                          θ                      +                                                                        4                          3                                                ⁢                        π                                                              )                                                                                                      sin                ⁢                                  xe2x80x83                                ⁢                θ                                                                                        -                                  sin                  ⁡                                      (                                          θ                      +                                                                        11                          6                                                ⁢                        π                                                              )                                                                                                      sin                ⁡                                  (                                      θ                    +                                                                  1                        2                                            ⁢                      π                                                        )                                                                    ]              ⁡          [                                    Iu                                                Iv                              ]      
where: Id and Iq are d- and q-axis detection current values; Iu and Iv are U-phase and V-phase detection current values to be inputted to the brushless motor; and xcex8 is an electrical angle responsive to a rotational position of the brushless motor.
It is preferable that, in the mentioned d-q coordinate inverse-transformation section, a following mathematic expression is used as a computing expression to perform the d-q coordinate inverse transformation:       [                                        Vu            *                                                            Vv            *                                ]    =                              2          3                    ⁡              [                                                            sin                ⁡                                  (                                      θ                    +                                                                  1                        2                                            ⁢                      π                                                        )                                                                                                      -                  sin                                ⁢                                  xe2x80x83                                ⁢                θ                                                                                        sin                ⁡                                  (                                      θ                    +                                                                  11                        6                                            ⁢                      π                                                        )                                                                                    -                                  sin                  ⁡                                      (                                          θ                      +                                                                        4                          3                                                ⁢                        π                                                              )                                                                                      ]              ⁡          [                                                  Vd              *                                                                          Vq              *                                          ]      
VW*=xe2x88x92Vu*xe2x88x92Vv*
where: Vu*, Vv* and Vw* are U-phase, V-phase and W-phase voltage command values; Vd* and Vq* are d- and q-axis voltage command values; and xcex8 is an electrical angel responsive to the rotational position of the brushless motor.
An electric power steering control method according to the invention arranged to control a drive current of a brushless motor that generates a steering assist force on the basis of a steering torque acting on a steering shaft and a vehicle speed, comprises:
a first step of computing a d- and q-axis current command value for generating a steering assist force in accordance with a predetermined characteristic on the basis of the mentioned steering torque and the mentioned vehicle speed;
a second step of performing a d-q coordinate transformation on the basis of a detection current signal in response to a three-phase AC current to be inputted to the mentioned brushless motor and an electrical angle signal in response to a rotational position of the mentioned brushless motor thereby obtaining ad- and q-axis detection current value;
a third step of computing a d- and q- axis voltage command value in response to a deviation between the mentioned d- and q-axis current command value and the mentioned d- and q- axis detection current value;
a fourth step of performing a d-q coordinate inverse transformation on the basis of the mentioned d- and q-axis voltage command value and the mentioned electrical angle signal, thereby computing a three-phase voltage command value;
a fifth step of performing compensation with respect to the mentioned three-phase voltage command value in accordance with a computing expression for improving efficiency of utilizing a previously set voltage, thereby generating a three-phase voltage command compensation value; and
a sixth step of generating a PWM waveform for PWM drive of the mentioned brushless motor in response to the mentioned three-phase voltage command compensation value.
As a result of such arrangement, -it is possible to achieve an electric power steering control method capable of performing normally any PWM drive even in the range that an amplitude of a fundamental wave of a line output voltage is not more than that of a power supply voltage, thereby utilizing the power supply voltage at the maximum.
It is preferable that, in the mentioned fifth step, a three-phase voltage command compensation value is obtained by subtracting a predetermined voltage value from the mentioned three-phase voltage command.
It is preferable that a value obtained by averaging a maximum value and a minimum value among the mentioned three-phase voltage command values is used as the mentioned predetermined voltage value.
It is preferable that a minimum value of the mentioned three-phase voltage command values is used as the mentioned predetermined voltage value.
It is preferable that, in the case of using the minimum value of the mentioned three-phase voltage command values as the mentioned predetermined voltage value, compensation is performed with respect to the mentioned three-phase voltage command value in the mentioned three-phase voltage command compensation section, only when revolution of the mentioned brushless motor is not less than a predetermined value.
It is preferable that, in the mentioned fifth step, a tertiary higher-harmonic of the mentioned three-phase voltage command value is superposed on the mentioned three-phase voltage command value, thereby obtaining a three-phase voltage command compensation value.
It is preferable that, in the case of obtaining a three-phase voltage command compensation value by superposing the tertiary higher harmonic of the mentioned three-phase voltage command value on the mentioned three-phase voltage command value, compensation is performed with respect to the mentioned three-phase voltage command value in the mentioned three-phase voltage command compensation section, only when the mentioned steering torque is not less than a predetermined value.
It is preferable that, in the mentioned second step, a following mathematical expression is used as a computing expression to perform the d-q coordinate transformation:       [                            Id                                      Iq                      ]    =                    2            ⁡              [                                                            -                                  sin                  ⁡                                      (                                          θ                      +                                                                        4                          3                                                ⁢                        π                                                              )                                                                                                      sin                ⁢                                  xe2x80x83                                ⁢                θ                                                                                        -                                  sin                  ⁡                                      (                                          θ                      +                                                                        11                          6                                                ⁢                        π                                                              )                                                                                                      sin                ⁡                                  (                                      θ                    +                                                                  1                        2                                            ⁢                      π                                                        )                                                                    ]              ⁡          [                                    Iu                                                Iv                              ]      
where: Id and Iq are d- and q-axis detection current values; Iu and Iv are U-phase and V-phase detection current values to be inputted to the brushless motor; and xcex8 is an electrical angle responsive to a rotational position of the brushless motor.
It is preferable that, in the mentioned fourth step, a following mathematic expression is used as a computing expression to perform the d-q coordinate inverse transformation:       [                                        Vu            *                                                            Vv            *                                ]    =                              2          3                    ⁡              [                                                            sin                ⁡                                  (                                      θ                    +                                                                  1                        2                                            ⁢                      π                                                        )                                                                                                      -                  sin                                ⁢                                  xe2x80x83                                ⁢                θ                                                                                        sin                ⁡                                  (                                      θ                    +                                                                  11                        6                                            ⁢                      π                                                        )                                                                                    -                                  sin                  ⁡                                      (                                          θ                      +                                                                        4                          3                                                ⁢                        π                                                              )                                                                                      ]              ⁡          [                                                  Vd              *                                                                          Vq              *                                          ]      
VW*=xe2x88x92Vuxe2x88x92Vv*
where: Vu*, Vv* and Vw* are U-phase, V-phase and W-phase voltage command values; Vd* and Vq* are d- and q-axis voltage command values; and xcex8 is an electrical angel responsive to the rotational position of the brushless motor.